fix(http): check response body text against undefined (#13017)

modules/@angular/http/src/body.ts

@@ -47,7 +47,7 @@ export abstract class Body {
       return String.fromCharCode.apply(null, new Uint16Array(<ArrayBuffer>this._body));
     }
 
-    if (this._body === null) {
+    if (this._body == null) {
       return '';
     }
 

modules/@angular/http/test/static_request_spec.ts

@@ -88,5 +88,14 @@ export function main() {
 
       expect(req.text()).toEqual('');
     });
+
+    it('should return empty string if body is undefined', () => {
+      const reqOptions = new RequestOptions(
+          {url: 'test', method: 'GET', headers: new Headers({'content-type': 'application/json'})});
+      delete reqOptions.body;
+      const req = new Request(reqOptions);
+
+      expect(req.text()).toEqual('');
+    });
   });
 }